This invention relates to automatic clothes dryers and more specifically to control systems therefor designed to initiate termination of the dry cycle when the percentage of moisture retained in the clothes load has declined to a level of dryness desired by the user.
It has been common practice to provide automatic clothes dryers with an electromechanical timer which the user manipulates to set the drying time at a value expected to be adequate for drying the clothing load. Although the timer dial is generally provided with markings to assist the user in selecting the proper amount of drying time, the user typically must estimate, based on experience, the amount of time needed to dry a particular load. Such estimating generally results in clothes being over dried which results in excess energy consumption and possible damage to the clothing, or under-drying, in which case the dryer is cycled on and off by the user with the load being inspected after each drying period to determine when the load is sufficiently dry.
In an effort to overcome the shortcomings of straight time setting, it has become common practice to provide for a certain degree of automatic control based upon load dryness. One such approach employs an interdigitized resistance member positioned so as to intermittently be contacted by the clothes as they tumble. The resistance member is coupled to a capacitor, which is charged by the control circuit at a certain rate and is discharged through the resistance member at a rate determined by the degree of dryness of the clothes contacting the member. When the capacitor becomes charged to a certain level, a timer which is set by the user is activated. The dry cycle is terminated when the timer times out. When very wet, the clothes present a relatively low resistance rapidly discharging the capacitor. As the clothes become drier, contact by the clothes discharges the capacitor at a decreasing rate. Thus, the timer is cycled intermittently as a function of the dryness of the clothes. As the clothes become dryer, the ON time of the timer increases until the timer times out ending the cycle.
Another approach involves monitoring outlet air temperature from the drum and energizing a timer when the sensed temperature reaches a predetermined level. Both of these approaches still requires the user to make some estimate of load size.
Yet another approach which purports to eliminate the need for estimating load size is taught by U.S. Pat. No. 4,109,259 which employs a thermostatic switch responsive to the cooling effect of the clothing load. The duty cycle of the switch is varied to provide a shorter dryer time for small loads than for larger loads. Electromechanical arrangements of this type are subject to long term reliability problems associated with repeated operation. Accuracy of control can also be a problem dependent upon the closeness with which the thermostatic switches can be manufactured to design specifications.
Commonly assigned U.S. Pat. No. 4,286,391 discloses a microprocessor based control system which senses outlet air temperature and advantageously uses the fact that for a constant inlet air temperature the outlet air temperature is substantially constant over a major portion of the dry cycle at an absolute value dependent upon the size of the clothes load and which rises near the end of the cycle. The microprocessor is programmed to detect the constant temperature and initiate termination of the dry cycle when the sensed temperature exceeds the constant temperature by a predetermined percent of the constant temperature. The percentage corresponding to the user selected dryness level. While this approach overcomes problems associated with the electromechanical systems mentioned hereinbefore, it employs absolute temperature measurement. For satisfactory operation, relatively high accuracy temperature sensing components are needed. Components providing such accuracy capability are costly.
It is therefore an object of this invention to provide a control system which obviates the problems associated with electromechanical controls and which does not require costly high accuracy temperature sensing components.
It is a further object of the invention to provide a dryer control system which initiates termination of the dry cycle based upon relative measurements rather than absolute measurements.